The present invention relates to a method and/or architecture for electronic signal amplification generally and, more particularly, to electronic control of an amount of signal amplification, or gain, provided by a variable gain amplifier (VGA).
High performance amplifiers such as those used in the front end of broadband and wireless communication receivers require excellent noise, linearity and bandwidth performance. The performance of the overall system benefits greatly if the front end amplifier can vary the signal gain as the strength of the received signal changes. However, maintaining noise performance, linearity performance and high frequency or broadband operation is difficult in variable gain amplifier (VGA) designs. Even amplifiers with less stringent performance requirements suffer from difficulties in controlling variable gain, since the amount of gain that must be varied (i.e., dynamic range) can often be large. Furthermore, a specific gain-versus-control signal characteristic is often desired (i.e., xe2x80x9clinear-in-dBxe2x80x9d transfer functions; which are characterized by an exponential change in amplifier gain for a commensurate linear change in control signal).
Referring to FIG. 1, a conventional circuit 10 is shown illustrating a master-slave gain control scheme for variable gain amplifiers. The circuit 10 comprising a variable amplifier 12, a variable amplifier 14 and a control amplifier 16 is shown. A signal IN is presented to the variable amplifier 12 and presented as a signal OUT. The amplifiers 14 and 16 control the variable gain of the amplifier 12.
Currently, most front end amplifiers in high frequency RF receivers (i.e., low-noise amplifiers (LNAs)) have a fixed amount of gain. LNAs with variable gain usually change gain in discrete steps based on a digital control signal (i.e., high-gain mode and low-gain mode.) Some high performance amplifiers use the exponential voltage-current transfer function of bipolar junction transistors (BJTs) to provide linear-in-dB control characteristics by varying the bias of a BJT based amplifier. Other implementations use a master-slave control scheme as illustrated in FIG. 1 to maintain linear-in-dB control (to avoid changing the bias current in the high performance amplifier). Master-slave control schemes have the added advantage of providing preconditioning to the control signal (i.e., temperature compensation, etc.).
Referring to FIGS. 2(a-d), various conventional VGA topologies are shown, each illustrating different gain varying elements and techniques. Referring to FIGS. 3(a-b), various implementations of conventional VGA topologies are shown. FIG. 3b illustrates a conventional VGA with multiple gain elements.
VGAs that change gain in discrete steps can not take full advantage of the signal-to-noise benefits of matching changes in received signal strength with commensurate changes in amplifier gain. Furthermore, control schemes that use multiple digital control signals to provide relatively small discrete gain steps suffer from complicated and undesirable interfaces between the VGA and the additional digital controller. VGAs based on the exponential function of the BJTs suffer from degraded linearity performance when the bias current of the BJT is low. Control schemes which do not rely on the BJT exponential characteristic vary amplifier gain using a variable-resistance, typically as a FET transistor. FET control schemes require a master-slave control loop to achieve a linear-in-dB characteristic. However, FET transistors can also suffer from degraded linearity performance as variable resistance is increased. Moreover, all the control schemes illustrated suffer from limited dynamic range.
The present invention concerns an apparatus comprising a first circuit and a second circuit. The first circuit generally comprises one or more master amplifiers and a plurality of control amplifiers. The first circuit may be configured to generate a plurality of control signals in response to (i) a first signal related to a desired gain and (ii) a second signal related to a known reference. The second circuit may be configured to generate an output signal in response to (i) an input signal and (ii) the plurality of control signals. The output signal may be amplified with respect to the input signal.
The objects, features and advantages of the present invention include providing a method and/or architecture for electronic control of an amount of signal amplification, or gain, provided by a variable gain amplifier (VGA) that may (i) control multiple control elements, (ii) control multiple VGA stages, (iii) provide a continuous (non-discrete) gain characteristic and/or (iv) improved linearity, noise, bandwidths and dynamic range of the amplifier.